For soliton transmission with in-line amplification (by EDFA), the problems that remain to be solved are known:
1) Gordon-Haus jitter which causes uncertainty concerning the arrival times of signal bits; and PA1 2) the accumulation of noise due to spontaneous emission being amplified in the optical amplifiers. PA1 1) synchronous modulation with filtering, D1=H. Kubota and M. Nakazawa, (1983), "Soliton transmission control in time and frequency domains", ILS J. Quantum Electronic, Vol. 29, No. 7, pp. 2189-2197, July 1993, demonstrates the theoretical advantage of the method by calculation. No practical solution is recommended, but reference is made to an experimental transmission at 10 Gbit/s over 1 million kilometers in D2=Nakazawa et al. (1991), "Experimental demonstration of soliton data transmission over unlimited distances with soliton control in time and frequency domains", Electronics Letters, Vol. 29, No. 9, pp. 729-730, Apr. 29, 1993.
Various solutions have been proposed and described in the following documents which are expressly incorporated in the present application as descriptions of the prior art:
Document D2 teaches the use of an LiNO.sub.3 optical modulator for performing synchronous modulation of solitons, as shown diagrammatically in FIG. 2. The problem with that solution is that the signal rate to be regenerated cannot exceed 20 to 30 Gbit/s (10 Gbit/s in document D2). The LiNO.sub.3 modulator MOD is controlled by an electronic control signal generated in a clock circuit from the in-line soliton signal. The clock recovery means comprise: an optical coupler C5 for extracting a portion of the optical signal propagating between the inlet F1 and the outlet F2; a clock extractor circuit CLKX; a delay line for providing a delay DEL; and an amplifier GM for providing the control power necessary for operating the LiNO.sub.3 modulator MOD. FIG. 2 also shows: an inlet optical amplifier (EDFA) for overcoming the insertion losses of the regenerator circuit; birefringent polarization control devices (PC); and a bandpass filter BP for narrowing the spectral distribution of energy in the solitons. Some of those accessories are to be found in the preferred embodiments of the invention as described below.
A system is also known for regenerating an optical signal in the form of a train of bits constituted by light pulses for "1" bits and by lack of pulses for "0" bits at predetermined moments known to a clock: D3=J. K. Lucek and K. Smith, (1993), "All-optical signal regenerator", Opt. Lett., Vol. 18, No. 15, pp. 1226-1228, Aug. 1, 1993.
Other documents are helpful in obtaining a better understanding of the present invention and are presented briefly with a summary of their contributions for this purpose. Those documents are also expressly incorporated in the present application as descriptions of the prior art:
D4=K. Smith and J. K. Lucek, (1992) "All-optical clock recovery using a mode locked laser", Elect. Lett. 28 (19), p. 1814, Sep. 10, 1992. That document describes all-optical clock recovery from a soliton signal by locking the modes of an optical fiber ring laser by injecting said soliton signal into the ring.
D5=L. E. Adams et al., (1994) "All-optical clock recovery using a mode locked figure-eight laser with a semiconductor nonlinearity", Electron. Lett., Vol. 30, No. 20, pp. 1696-1697, Sep. 29, 1994. That document teaches the use of a mode locked laser for all-optical clock recovery. All-optical clock recovery can be used in soliton regenerator apparatus using an optical modulator of the invention to provide the control signal(s) of said optical modulator.
D6=P. L. Frangois and T. Georges, (1993) "Reduction of averaged soliton interaction forces by amplitude modulation", Optics Lett. 18 (8), p. 583, Apr. 15, 1993. That document describes comparison by computer simulation of three methods of soliton signal modulation: 1) amplitude modulation only; 2) applying alternating phases (.+-..pi.) to successive solitons; and 3) amplitude modulation with the application of alternating phases to successive solitons. The first method is effective only for resetting the time positions of soliton pulses for the purpose of eliminating Gordon-Haus jitter. The second method is effective for obtaining a reduction in interaction forces (collisions) between adjacent solitons on the propagation waveguide. The third method obtains simultaneously the advantages of both the preceding methods.
D7=K. Uchiyama et al., (1992) "Ultrafast polarization-independent all-optical switching using a polarization diversity scheme in the nonlinear optical loop mirror (NOLM)", Electron. Lett., Vol. 28, No. 20, pp. 1864-1866, Sep. 24, 1992. That document shows the use of a NOLM as a switch, which is made insensitive to the polarization of the light in the signal to be switched. This is obtained by using a polarization-maintaining fiber which is cut and twisted through 90.degree. at the midpoint of the NOLM loop. The principle thereof is shown in FIG. 4.
By way of example, the loop of the NOLM is constituted by a PANDA polarization-maintaining fiber having two holes. By 90.degree. rotation between the axes A1 and A2 halfway along the propagation path, the fast axis of the lefthand portion becomes the slow axis of the righthand portion of the loop shown in FIG. 4 (and the left slow axis becomes the right fast axis). The fiber of the loop (L) is polarization dispersive, i.e. the speed of light propagation within the fiber is different for polarization that is in alignment with the fast axis and for polarization that is orthogonal to the fast propagation axis, i.e. the slow axis of the fiber. It is necessary to overcome polarization dispersion, and this is done by using two identical lengths of fiber having their polarization-maintaining axes A1 and A2 in a mutually orthogonal disposition, thus having the effect of cancelling polarization dispersion.
To make the system independent of the polarization of the switched signal, the polarization of the control signal which is injected into the loop L via the coupler C2 on the control inlet fiber F3 is injected at 45.degree. to the two orthogonal axes A1 and A2. In the same manner as above, the effects of polarization dispersion cancel.
D8=French patent application FR 94/15555 of Dec. 23, 1994 in the name of Alcatel N. V. and entitled "Dispositif de regeneration en ligne d'un signal transmis par solitons via la modulation synchrone des solitons a l'aide d'un miroir optique non-lineaire" Apparatus for in-line regeneration of a signal transmitted by solitons by synchronous modulation of the solitons using a non-linear optical mirror!. In the opinion of the Applicant, that document, still unpublished at the date of the present application, is the closest document in the prior art for assessing the contribution of the present invention.
The apparatus described in D8 regenerates solitons in line by synchronous modulation of the solitons using a non-linear optical mirror (NOLM) as an optical modulator, the NOLM modulator being controlled by a clock signal recovered from the soliton signal by clock recovery means which may be all-optical means or electro-optical means. The NOLM includes an inlet coupler C1 which may be a 50/50 coupler or an asymmetrical coupler. Said inlet coupler C1 may be a 2.times.2 or a 3.times.3 coupler. In a particular embodiment, the regenerator system also includes a plurality of optical amplifiers and a plurality of filters referred to as "guiding" filters. As in the context of the present invention, the intended application is optical telecommunications over great distances by means of solitons.
Like D4, document D8 teaches the use of optical clock recovery by locking the modes of a loop fiber laser in a NOLM operating independently of the polarization of the control signal in application of the teaching of D7, but used as an optical modulator and not as an optical switch (as disclosed in D3 and D7).
The optical modulator taught by document D8 is mainly an amplitude modulator, but inevitably it introduces a component of phase modulation of greater or smaller magnitude, depending on the relative amplitude and waveform of the control signal injected into the NOLM. Said phase modulation is not detrimental in itself, but since it is not possible to optimize it independently, the phase modulation introduces a "chirp" effect, meaning that the amount of phase modulation depends on the frequency of the modulated wave, thus having a harmful effect on the spectral composition of solitons modulated in this way.
D9=Hak Kyu Lee et al., "A walk-off balanced NFLM switch controlled by 1.5 .mu.m pulses for high bit rate 1.3 .mu.m telecommunications", Proc. 21st Eur. Conf. on Opt. Comm. (ECOC '95), Brussels, p. 401. Document D9 teaches the use of two control inlets in a NOLM to avoid phase slip ("walk off") between the control signal and the signal to be switched. In that case the NOLM acts only as a switch (as in D3 and in D7) and not as a modulator (as in D8 and in the present invention). The clock signals presented to the control inlets are not phase-shifted as in that prior art.